1. Field of the Invention
The present invention relates to a numerical control apparatus of a machine tool, for example, a machining center and a control method of the machine tool.
2. Description of the Related Art
In a machining program prepared for machining a workpiece used in a numerical control apparatus (NC apparatus) of for example a machining center or other machine tool, commands for defining linear movement of a cutting tool to the workpiece include a rapid feed command and a cutting feed command.
A rapid feed command is normally a command for positioning the workpiece and the cutting tool when not cutting the workpiece by the cutting tool. Accordingly, it is not necessary to consider the effect of vibration occurring due to the movement and the path precision of a machining point of the cutting tool exerted upon the machined surface of the workpiece. When using a rapid feed command, it is possible to move the cutting tool at a high feed speed and a high acceleration.
On the other hand, a cutting feed command is a command used predicated on actual machining of the workpiece by the cutting tool. Accordingly, it is necessary to consider the effect of the vibration occurring due to the linear movement and the path precision of the machining point of the cutting tool exerted upon the machined surface of the work. For this reason, in an NC apparatus, the feed speed of the tool when using a cutting feed command is restricted in comparison with a rapid feed command from the viewpoint of maintaining the machining precision etc.
In machining centers and other machine tools, there is a strong demand for shortening the machining time by realizing high speed machining of a workpiece.
For example, a free curved surface of a die or the like is cut by reciprocal movement of an end mill T as shown in FIG. 1. Specifically, the end mill T is moved linear in the X-direction by the cutting feed command and stopped when it reaches a target position. Then, a pick feed is performed to move the end mill T linear in the Y-axis direction at a feed pitch P by a rapid feed command. Further, the end mill T is moved in a reverse direction in the X-axis direction by a cutting feed command.
However, when cutting a free curved surface by the process as described above, it is necessary to repeatedly stop the end mill T for the pick feed. For this reason, there were the disadvantages of abrupt acceleration and deceleration for the positioning of the end mill T, a susceptibility to shock, a long time required for the pick feed, and as a result a long machining time.
As a method for solving this, it has been proposed not to perform the pick feed of the end mill T by linear movement, but to make the path of movement of the end mill T curved or arc like such as the path shown by the broken line in FIG. 1. By making the path of movement of the end mill T an arc or other curve, it is possible to continuously and smoothly shift from cutting feed in the X-axis direction to pick feed and from pick feed to cutting feed in the X-axis direction in a reverse orientation to the former, so the shock can be eased.
In a conventional NC apparatus, in order to perform this pick feed by an arc or other curved path of movement, it is necessary to use an arc cutting feed command. The only arc or curve feed command standardized as a preparation function (G function) of a numerically controlled machine tool in Japanese Industrial Standards (JIS) B6314 is an arc cutting command. For this reason, as described above, there was the disadvantage that the feed speed in the pick feed was restricted, so it was difficult to shorten the machining time.
On the other hand, recently, machining programs for cutting as described above are being automatically programmed by computer aided design (CAD) systems and computer aided machining (CAM) systems. If such machining programs are executed by machining centers having different machining performances, the precision of the machined surface of the workpiece becomes different according to the machining performance. In order to keep the precision of the machined surface constant even if the machining performances of the machining centers used are different, it is necessary to correct and adjust the feed speed in the machining program at the NC apparatus side of each machining center, so there is the disadvantage that a great amount of trouble is involved.
An object of the present invention is to provide a numerical control apparatus of a machine tool capable of shortening the machining time while maintaining the machining precision of the work.
Another object of the present invention is to provide a numerical control apparatus of a machine tool capable of maintaining the machining precision of a workpiece even if a workpiece is machined by machine tools having different performances using the same machining program.
Still another object of the present invention is to provide a control method of a machine tool using the numerical control apparatus.
According to a first aspect of the present invention, there is provided a numerical control apparatus of a machine tool comprising an analyzing means for analyzing a machining program defining machining process of a workpiece by the machine tool to extract from, a cutting feed command program included in the machining program, information for specifying a feed speed of a cutting tool to the workpiece and cutting/non-cutting information defining actual cutting by the cutting tool added to the cutting feed command program, a feed speed optimizing means for calculating a permissible feed speed of the cutting tool in accordance with the content of the cutting/non-cutting information based on the extracted information and optimizing the feed speed of the cutting tool to be within the permissible feed speed, and a distributing means for calculating control commands to be distributed to the control axes of the machine tool based on the optimized feed speed.
According to a second aspect of the present invention, there is provided a numerical control apparatus of a machine tool comprising an analyzing means for analyzing a machining program defining machining process of a workpiece by the machine tool to extract from, a cutting feed command program included in the machining program, information for specifying a feed speed of a cutting tool to the workpiece and permissible error defining a permissible range of error from the path of movement of the cutting tool added to the cutting feed command program, a feed speed optimizing means for calculating a permissible feed speed of the cutting tool in accordance with the permissible error based on the extracted information and optimizing the feed speed of the cutting tool to be within the permissible feed speed, and a distributing means for calculating control commands to be distributed to the control axes of the machine tool based on the optimized feed speed.
According to a third aspect of the present invention, there is provided a numerical control apparatus of a machine tool comprising an analyzing means for analyzing a machining program defining machining process of a workpiece by the machine tool to extract from, a cutting feed command program included in the machining program, information for specifying a feed speed of a cutting tool to the workpiece and machining information added to the cutting feed command program, a feed speed optimizing means for optimizing a feed speed of the cutting tool based on the extracted information, and a distributing means for calculating control commands to be distributed to the control axes of the machine tool based on the optimized feed speed.
According to a fourth aspect of the present invention, there is provided a control method of a machine tool comprising steps of preparing a machining program defining machining process of a workpiece by the machine tool provided with a cutting tool able to move in the directions of a plurality of control axes by a driving means, adding cutting/non-cutting information defining actual cutting by the cutting tool to a cutting feed command program included in the machining program, downloading the machining program added with the cutting/non-cutting information to a numerical control apparatus and executing the machining program, extracting at the numerical control apparatus, from the cutting feed command program, information for specifying a feed speed of the cutting tool to a workpiece and cutting/non-cutting information, calculating the permissible feed speed of the cutting tool in accordance with the content of the cutting/non-cutting information based on the information for specifying the feed speed of the cutting tool and optimizing the feed speed of the cutting tool to be within the permissible feed speed, and calculating control commands to be distributed to the control axes of the machine tool based on the optimized feed speed and controlling the control axes.
According to a fifth aspect of the present invention, there is provided a control method of a machine tool provided with a numerical control apparatus for driving a machine tool to execute cutting of a workpiece in accordance with a machining program, comprising steps of preparing a machining program defining machining process of a workpiece by the machine tool, adding information of a permissible error defining a permissible range of error from a path of movement of a cutting tool to a cutting feed command program included in the machining program, downloading the machining program added with the information of the permissible error to a numerical control apparatus and executing the machining program, extracting at the numerical control apparatus, from the cutting feed command program, information for specifying a feed speed of the cutting tool to the workpiece and the information of the permissible error, calculating the permissible feed speed of the cutting tool in accordance with the permissible error based on the extracted information and optimizing the feed speed of the cutting tool to be within the permissible feed speed, and calculating control commands to be distributed to the control axes of the machine tool based on the optimized feed speed and controlling the control axes.
According to a sixth aspect of the present invention, there is provided a control method of a machine tool provided with a numerical control apparatus for driving a machine tool to execute cutting of a workpiece in accordance with a machining program, comprising steps of preparing a machining program defining a machining process of a workpiece by the machine tool, adding predetermined machining information to a cutting feed command program included in the machining program, downloading the machining program added with the machining information to a numerical control apparatus and executing the machining program, extracting at the numerical control apparatus, from the cutting feed command program, information for specifying a feed speed of the cutting tool to a workpiece and the machining information, optimizing the feed speed of the cutting tool based on the extracted information, and calculating control commands to be distributed to the control axes of the machine tool based on the optimized feed speed and controlling the control axes.
In the present invention, when preparing a machining program, machining information is newly added to the cutting feed command program contained in the machining program.
When the machining program with this machining information added is executed, information specifying the feed speed of the cutting tool to the workpiece is extracted from the cutting feed command program contained in the machining program and the machining information added to the cutting feed command program is extracted.
Based on the extracted information, the feed speed of the cutting tool is optimized. Namely, the feed speed is determined based on the machining information added to the cutting feed command program.
For example, by adding cutting/non-cutting information defining actual cutting by the cutting tool as the machining information, the feed speed is restricted when actually cutting so as to maintain the precision of the machined surface. When not cutting, the feed speed is set at a speed as high as possible in order to realize high speed machining.
Further, by adding information of a permissible error from the path of movement of the cutting tool to the cutting feed command program, the feed speed is optimized so as to achieve the machining precision defined by the permissible error.
That is, the feed speed is optimized in accordance with the performance of the machine tool, for example, a position loop gain of a servo controlling means. Therefore, even if the same machining program is executed in machine tools having different performances, the machining precision is maintained within a range of the permissible error.